(a) Field of the Invention
The present invention relates to an optical repeater having independently controllable amplification factors and, more particularly, to an optical repeater for amplifying optical signals transmitted through a pair of optical fibers.
(b) Description of the Related Art
In an optical fiber communication system for transmitting optical signals, a plurality of signal amplifiers are provided as optical repeaters at a constant pitch for compensating the optical loss generated in the optical fiber. The signal amplifiers used in the optical fiber communication system are categorized into two types: an electric amplifier which converts the optical signal to an electric signal, amplifies the electric signal and re-converts the amplified electric signal into an optical signal; and an optical amplifier which amplifies the optical signal as it is.
In particular, the optical amplifiers are used as optical repeaters for an undersea optical cable because such optical repeaters are requested to have lower power dissipation. Known optical amplifiers include an optical fiber amplifier which uses an optical fiber having a core doped with rare earth elements, and a Raman amplifier which uses the principle of Raman scattering generated by radiating a pump light onto the optical fiber transmitting the optical signals to be amplified.
FIG. 6 shows a conventional optical repeater described in a publication entitled “Erbium-doped optical fiber amplifier” by Syouich Sudo in NTT Corp., published from Opto-Electronics Co. The optical repeaters i.e., optical fiber amplifier is used for amplifying uplink signal and downlink signal transferred through an optical fiber cable, and includes therein a wavelength-division-multiplexing (WDM) optical couplers 10, optical isolators 30, pump laser diodes (LD) 40, wavelength synthesis couplers 60, erbium-doped fibers (EDFs) 76, and a LD driver 75.
The optical isolators 30 are inserted in the uplink and downlink optical fibers 71 and 72 for suppressing the reflected light from transferring in the reverse direction. Pump laser diodes 40 emit pump lights having different central wavelengths of 1480-nm-band for amplifying the WDM optical signals. The LD driver 75 drives the pump laser diodes 40 while controlling the output powers of the pump laser diodes 40, thereby controlling the amplification factors of the optical signals transferred through the uplink and downlink optical fibers 71 and 72.
Both the optical couplers 60, i.e., tapping couplers 60 inserted in the uplink and downlink optical fibers 71 and 72 tap the optical signals then transferring through the uplink fiber 71 onto the downlink fiber 72, and tap the optical signals then transferring through the downlink fiber 72 onto the uplink fiber 71. This configuration allows the intensity of the optical signals transferring through the downlink fiber 72 to be monitored at the receiving end of the uplink fiber 71, and allows the intensity of the optical signals transferring through the uplink fiber 71 to be monitored at the receiving end of the downlink fiber 72.
The wavelength synthesis coupler 50 is a 2-input/2-output optical coupler, which couples together the pump lasers emitted to by two laser diodes 40 and outputs the coupled pump lasers to the EDFs 76 through both the WDM couplers 10 provided on the uplink and downlink fibers 71 and 72. The EDF 76 is an optical fiber having a core doped with erbium ions, or rare-earth elements, and receives the pump light through the WDM coupler 10 to amplify the optical signals transferring through the optical fiber 71 or 72.
In order for broadening the band of the optical signals to be amplified, it is effective to increase the number of laser diodes 40 having different central wavelengths. For example, if the optical repeater has four laser diodes 40 having different central wavelengths, each two of the four diodes are paired, the outputs of each pair are coupled by a wavelength synthesis coupler, and the two coupled lasers are again coupled by another wavelength synthesis coupler. This allows a pump light obtained from four laser diodes to be supplied to the EDFs 76 through the WDM couplers 10. The WDM coupler 10 operates for backward pumping, wherein the pump light is introduced to the EDF 76 in the direction opposite to the transfer direction of the optical signals through the uplink or downlink fiber.
The optical repeater may be replaced by a Raman amplifier which uses the principle of Raman scattering in the optical amplification. In such a case, the pump light supplied from the WDM coupler 10 is directly introduced to the uplink and downlink fibers 71 and 72 without using the EDFs 76.
In a recent optical communication system, along with the development of the WDM technique and increase of the number of multiplexed wavelengths in the optical signals, the optical amplifier is ever requested to have a broader-band characteristic. For meeting such a request, intensive studies are conducted for developing the optical repeaters having Raman amplifiers using 1480-nm-band pump lasers.
The optical repeater used for the undersea optical cable should have a higher reliability in addition to the lower power dissipation. However, in the conventional optical repeater such as described in the publication, if the pump light is not supplied to one of the WDM couplers 10 which supplies the pump light to the uplink fiber 71, the optical signals transferring through the downlink fiber 72 cannot be also amplified. In other words, a single failure disables the functions of the optical repeater for amplifying the optical signals transferring through both the uplink and downlink fibers. This degrades the reliability of the conventional optical repeater.
In addition, the conventional optical repeater does not have different amplification factors independently controlled for the uplink fiber and the downlink fiber, because the WDM couplers 10 receive the optical outputs of the same output power from the wavelength synthesis coupler 50. For example, if optical repeaters are disposed in number of 200 to 250 at a pitch of 40 to 50 km for an optical fiber cable having a transmission length of 10,0000 km, there may arise an imbalance of the optical signal power between the receiving end of the uplink fiber and the receiving end of the downlink fiber.
In particular, if a Raman amplifier is employed as the optical amplifier, the amplification factor for the optical signal has a considerable range of variation due to the characteristics of the optical fiber etc. In such a case, there arises a problem in that the intensity and quality of the optical signal is lowered unless the amplification factors of the optical repeaters are suitably controlled for the respective uplink and downlink optical fibers.